Physical and genetic studies of new and previously obtained deletion mutants of SV40 are continuing with the purpose of completing our characterization of the coding and regulatory regions of the SV40 DNA. We are particularly interested in clarifying the function of the region between 0.54 to 0.76, the region containing the origin of DNA replication, and, very likely, the sites for transcription initiation and/or processing of early and late messenger RNAs. Studies are also continuing to utilize varying segments of the SV40 and polyoma DNA to propagate pro- and eukaryote DNA segments in monkey and mouse cells. BIBLIOGRAPHIC REFERENCES: Biochemical Pastimes....and Future Times. Paul Berg. The Eighth Feodor Lynen Lecture, 9th Miami Winter Symp. Jan. 1977, in press. Mapping the Mammalian Genome. Paul Berg. Presented at the NAS Forum on Recombinant DNA Research, March 7-9, 1977, in press.